At my wit's end, 11 month old refuses to leave apartment for a walk with one person

Hello everyone,

I guess this post is a bit of everything so I was not sure which flair to use but I hope this is okay.

So me and my partner have an 11 month old Bernese Mountain Dog intact male puppy. My partner goes to the office and I don't so I'm the one who does most of the looking after and walking.

Now the issue is the puppy does not want to leave our apartment door for a walk with just one person meaning either me or my partner, when we go together he has absolutely no problem getting out the door whatsoever and follows us everytime. The only thing I can think of that could have impacted this behaviour is at the end of summer we took a 3 week road trip when the 3 of us spent the whole time together and also when we got home from that same trip we found someone had placed a big plant near the elevator. Before that he was absolutely fine going outside with either of us.

I have seriously started dreading having to take the dog out when my partner is at work as this has been going on for several months now... I know the puppy is at this age when they go through various fear stages which I thought it was at first but now I doubt it since he has no problem following both me and my partner when we go out together and also once out he has no problem doing all his business but when it's one of us for some reason the doorway is a huge problem... Please note our beast weighs over 40 kg (90 lbs for American friends) so simply picking him up and bringing him outside is not an option.

I have tried luring him with various high value treats nicely, dropping them outside the door and waiting for him to come out, most of the time he's not even interested and just stands at the doorway and keeps looking out at hell knows what, sometimes it almost works because he reluctantly gets the treats but when he notices me move towards the door to close it he immediately bolts inside before I can manage to do so... Earlier today I spent over 30 minutes waiting, dropping treats, praising when he moved even a tiny bit towards me but he just got the treats and ran back inside everytime.. I'm just so exhausted I ended up locking myself alone in a room and crying out of frustration all the while the dog was whining outside the closed door of the room I was in. Of course after that I ended up having to drag him out the door by the collar as usual with much fighting from his side and trying to slip out of his collar and I really don't want to do this but it's not like I have any choice...

I'm really trying but I'm just so exhausted I admit to at times having lost my temper and yelled at him or spoke too harshly (I have never hit him) but I feel like this damn puppy is testing me. I'm afraid all this has caused more issues but I just don't know what to do anymore. I really do love this dog and he brings so much joy but I'm about to lose my mind over this one issue. I wish he would just go out the damn door without any tantrums and such.

Thank you if you have managed to read this far and if you have any words of advice at all please please it's very much appreciated.